Custom Form Request Validation with unique validation not working on update

I'm not sure what I'm doing wrong, but I have a custom form request validation that I'm using in for Create and Update record with unique column validation. It's working fine for creating new record, but not for updating a record.

Custome Form Request

<?php
namespace App\Http\Requests;
use Illuminate\Foundation\Http\FormRequest;
class ServiceTypeRequest extends FormRequest
{

public function authorize()
{
    return true;
}

/**
 * Get the validation rules that apply to the request.
 *
 * @return array
 */
public function rules()
{
    return [
        'service_name'        => ['required', Rule::unique('service_type', 'Service')->ignore($this->service_type) ],
        'type'                => ['required', 'string'],
        'view_availability'   => ['required', 'boolean'],

    ];
 }
}

Controller Update

public function update(ServiceTypeRequest $request, ServiceType $serviceType)
{
    $validated = $request->validated();

    $service_type = ServiceType::update([
        'Service'               => $validated['service_name'],
        'type'                  => $validated['type'],
        'view_availability'     => $validated['view_availability'],
    ]);

    return redirect()
            ->route('service_type.index')
            ->with('status', 'Service type updated!');
}

Getting error when I submit the update form with PUT method It's complaining about the $this I have this inside the custom form validation for service_name.

Error
Using $this when not in object context
http://localhost:8021/service_type/58 

source https://stackoverflow.com/questions/70419652/custom-form-request-validation-with-unique-validation-not-working-on-update